In any electrochemical reaction, the driving force, i.e. the voltage, is greater than the theoretically necessary voltage and the additional amount of voltage required is known as an overvoltage. Many methods have been used in the past to reduce overvoltage including the use of depolarizers and various structural devices. The problem, however, is complicated because there is an overvoltage associated with each of the gases liberated during the electrochemical reaction. One such electrochemical energy storage system is based on electrolizing aqueous hydrogen chloride.
The use of carbon electrodes, i.e., electrodes manufactured from carbon, graphite, activated graphite, and the like is desirable because of a combination of their relatively low cost and availability. Unfortunately, such carbon electrodes have a significant chlorine overvoltage. In copending application Ser. No. 761,722, filed Jan. 24, 1977, a method of considerably and premanently decreasing the oxidation and reduction chlorine overvoltages of such carbon electrodes is disclosed. The method involves contacting the electrode with nitric acid for an effective overvoltage reducing length of time, generally 10 to 1200 hours, preferably 80 to 800 hours, and most preferably 400 to 700 hours. The nitric acid is preferably concentrated nitric acid and is most preferably boiling B-constant nitric acid. The disclosure of the copending application is hereby incorporated into this disclosure as if fully set forth here.
A means of further reducing the chlorine overpotential in an HCl electrochemical energy storage system in which the aqueous HCl electrolyte contains at least a portion of the HCl in disassociated form has been discovered and, further, new methods of storing the hydrogen and chlorine by-products of the system has also been discovered. Accordingly, it is the object of this invention to provide a means for decreasing the chlorine overvoltages in such a system and means for storing the hydrogen and chlorine by-products. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.